1. Field of the Invention
The invention relates to a circuit arrangement for detecting and digitizing an analog input signal having a first electronics unit to which the analog input signal is connected, having a second electronics unit for outputting a digital value representing the level of the analog input signal, and having an interface for galvanically separating the two electronics units. In addition, the invention relates to a field device for process instrumentation, where the field device includes such a circuit arrangement.
2. Description of the Related Art
Automation technology frequently employs field devices for process instrumentation, which are used, for example, for detecting and/or influencing process variables, and which are connected to each other via an automation network for exchanging data. Field devices that detect a physical or chemical quantity as a process variable are frequently referred to as transducers, because they convert the respective quantity into a measured value, which they output, for example, to a higher-order control station or as an actual value to a controller for further processing. Examples of such transducers are transducers for filling level, mass rate of flow, pressure, temperature, pH value or conductivity.
A field device for process instrumentation can be divided into a sensor that converts the physical or chemical quantity to be detected into an analog electrical input signal, a first electronics unit to which the analog input signal is connected, and a second electronics unit in which, for example, a characteristic curve calibration is performed and that is galvanically isolated from the first electronics unit by an interface.
After determining a digital value representing the level of the analog input signal, a measured value corresponding to it is output, for example, via a 4 to 20 mA interface with or without data communication according to the HART protocol or via another fieldbus, for example, PROFIBUS or PROFINET, for further processing within the automation system. If the auxiliary power required for operating the field device is provided via the communication interface of the second electronics unit, a portion of the auxiliary power must be routed via the interface for galvanic isolation, for example, with the aid of an isolation transformer or via capacitors, to the first electronics unit to supply the field device with power.
One problem when transmitting an analog input signal via a galvanically isolated interface is that a galvanically isolated transmission of an analog signal is always associated with a relatively high loss of accuracy. In order to circumvent this problem, it is possible, for example, to provide an analog/digital conversion in the first electronics unit with respect to a reference voltage, to convert the analog signal into digital form. The converted signal can then be routed to the second electronics unit in digital form via a serial interface of a microcontroller with the aid of an optocoupler or alternatively via a capacitive or inductive link. However, the use of such components would cause high power consumption in the first electronics unit and would likewise entail high component costs. There would also be the relatively high cost of developing suitable firmware for the microcontroller of the first electronics unit, on which high demands for reliability would be placed, because communication between the microcontroller in the first electronics unit and a microcontroller in the second electronics unit would always have to be maintained even if EMC disturbances occur. In addition, the firmware would have to ensure that current measured value data is always available.